I recently built a custom tank that has multiple pieces of cork making up a background and an island. I was not able to totally seal off behind the cork so in some areas there are rather large "caves". Do you think it is ok to leave these open or should I work to close them off? As of now the fish seem to be enjoying them.

Thanks!

PS, This is a fresh water community tank.

Depends what type of fish you have, and whether those fish are juveniles or adults.. In my experience, fish tend to get accustomed to their surroundings but do not realize how their own body changes through time or have a limited sense of self.. I once had this beautiful rock with holes in it.. I placed it in one of my aquariums.. the fish loved swimming through the holes, but a few of the species I had were juveniles and as they grew larger they began having trouble swimming through the holes, but they still tried to swim through the holes.. I knew it was time to remove the rock after having to dislodge a couple of my fish..

Hum,
That is a very good point. I think the openings are big enough to accommodate them even full grown.